A. Field of the Invention
The invention relates to the field of photographic computing devices, and in particular to devices for performing calculations useful in connection with darkroom procedures.
B. Prior Art
The primary component of any photographic system is a camera, which is used to record images on light-sensitive film contained therein. A typical camera comprises a body seal to block the entry of light therein except through a lens mounted on the front face of the body. Inside the body is a film holder assembly that is arranged to expose the film, a section at a time, to the light that enters the body through the lens. The entry of light through the lens into the body is controlled by shutter and diaphragm assemblies mounted in the body behind the lens. The shutter is designed to remain open only for a set, selected period of time. The diaphragm permits light to enter only through a set, selected portion of the diameter of the lens. In still cameras, the aperture is typically opened by a manually operated shutter release button, while cameras used for recording motion pictures contain automated shutter assemblies.
When light that is radiated and reflected from objects within the camera's field of view strikes the photosensitive emulsion of the film, the chemical characteristics of the emulsion are slightly altered in response thereto; the degree of alteration is a function of the intensity of the impinging light and the physico-chemical characteristics of the emulsion. The exposed film thus retains a "latent" image of the scene that appeared in the camera's field of view when the shutter was open.
The latent image must be amplified chemically and made permanent before it can be viewed. Slide film is developed directly into a positive image and thus emerges from processing in a completed state. Print film, on the other hand, yields a negative tonal image that must be reversed in order to render the recorded scene accurately. Typically, the image is reproduced on larger, light-sensitive substrates for viewing.
Darkroom photography consists of those procedures necessary to transform exposed film into finished prints or slides. Such procedures may be highly automated, as in the case of commercial film and slide processing, or performed manually to obtain especially high levels of quality or artistic control, or to produce special effects.
Darkroom procedures may be loosely organized into several categories: (1) film processing, which consists of subjecting the exposed film to chemicals that transform the latent image into a visible image and make this image permanent (the tonal rendition will be negative in the case of print film, and positive in the case of slide film); (2) print enlargement, wherein the negative is placed in an enlargement device that magnifies the image present on the negative and projects it onto a sensitized substrate (usually, but not always, light-sensitive photographic printing paper), thus producing a positive image; (3) print processing which, like film processing, involves subjecting the substrate exposed by the enlarger to chemicals that amplify the latent image produced by enlargement exposure so as to render it visible and permanent on the substrate; and (4) post-development processes, such as toning or archival processing, which produce further chemical and/or physical changes in the developed print to enhance appearance, permanence or other features of interest to the photographer.
Non-automated darkroom procedures call for mixing various solutions and performing a number of specialized procedures in a specified order. These procedures must be carried out in total darkness (where color processing or exposed but undeveloped black-and-white film is involved), under a "safelight" (which will not expose, and is therefore "safe," with respect to black-and-white print emulsions) or, least often, under ordinary lighting. Critical to successful completion of these procedures is strict control of parameters such as solution concentration, timing, temperature levels, and exposure.
Because of the limited life of photographic chemicals, darkroom photographers must constantly prepare fresh batches of solutions. This requires careful measurement of solid and liquid components. Although pre-mixed chemical concentrates typically require no more than addition of water, only the most popular formulas are obtainable in packaged form. Often, for reasons of economy or because of unavailability of pre-mixed chemicals, photographers consult published (or original) formulas and purchase the appropriate reagents in bulk. Such formulas frequently appear in exotic measurement units, which may differ from those of readily available measuring equipment or prove cumbersome to relate to other pertinent formulas.
Darkroom photographers must also contend with rather precise temperature requirements for various solutions. Stock solutions are typically stored and must be mixed at room temperature, often resulting in guesswork with respect to the temperature of a diluent such as water that may be heated or cooled easily. Even when the target temperature is reached, constant vigilance must often be maintained to prevent the solution from becoming hotter or colder, or at least to alter processing in response.
The film development process is particularly sensitive to changes in temperature. Because development temperature and proper development time are related to one another, it is possible to adjust the duration of processing rather than attempt to alter the temperature of the solution. The relationship between temperature and time is somewhat complex, however, and the necessary computations cannot be performed conveniently, particularly in a darkroom environment.
The enlargement process, like ordinary photography, presents the practitioner with several exposure parameters that are variable on a continuous or nearly continuous basis. Darkroom photographers usually settle on a single set of exposure parameters to produce an acceptable print by making "test strips," wherein different segments of photographic printing paper are exposed for different intervals of time or at different apertures in order to produce a selection of possible exposures. However, the exposure thus chosen will change if the photographer varies the aperture of the enlarger lens, the duration of exposure, raises or lowers the enlarger head, or inserts filters. Often, the photographer must produce another set of test strips for each parameter variation.
Very few photographic computational aids appear in the prior art, and even fewer pertain directly to darkroom photography. U.S. Pat. No. 4,498,140 discloses an electronic device for controlling the exposure of film to the various lights or lamps conventionally used in certain graphic arts processes. U.S. Pat. No. 4,120,038 describes an electronic calculator intended for use as a component of a flash unit, which computes a single dedicated variable (aperture) from flash and film sensitivity data only. U.S. Pat. No. 4,297,011 sets forth a system for calculating aperture and distance values dictated by an electronic flash, which system is integrated into the flash unit, and also displays the values so calculated.